1. Field of the Invention
The present invention pertains a recessed screw having a recess formed in the head thereof to receive a driver bit, more particularly relates to an improved shape of the recess, and further includes the driver bit for use with and engageable with the recessed screw having the improved recess.
2. Description of Prior Art
One of the prior art, recessed screws which is called a "Torx" ( a registered trademark, see Japanese Utility Model Publication No. Sho. 48-39959) does comprise a screw head 1 in which a recess 2 engageable with a driver bit is formed as shown in FIGS. 6 and 7. This recess is defined by semicylindrical concaves 3 and semicylindrical convexes 4 which alternate one with another, and each convex continues smoothly to the adjacent concaves. The concaves 3 are retracted radially and outwardly, with the convexes 4 protruding radially and inwardly so that they provide six grooves 5 and six thick vanes 6 to be driven. The side driven walls of each vane extend straight in parallel with an axis of the screw. Because of this feature, the Torx screws are advantageous in that there arises no problem of "ride-out" of the driver bit when it is driven, thus permitting a high torque to be applied to the recessed screw.
Contrary to usual screws each having a crossed recess, the Torx screws which lack the "biting" action for driver bits are disadvantageous in the automatic fastening of the screws. A clearance between the recess of Torx screw and the driver bit is so large that the bit is likely to slip off the recess. This is a serious problem in particular when a self-drilling screw or an elongate self-tapping one is fastened at high rotational speeds by a vibrating bit.
In general, the self-drilling and self-tapping screws are hardened before delivery to users. Thus, the driven vanes in their recesses are rendered harder and of a significantly higher mechanical strength as compared with non-hardened screws. On the other hand, the driver bit has driving vanes designed for engagement with the grooves 5 in the recess 2 of Torx screws. A cross-sectional area of the driving vane is therefore designed to match such an area "a" of groove 5, which however is much smaller than that "b" of each driven vane 6. Consequently, the driver bit will be worn soon if repeatedly used with the hardened screws of the self-drilling or self-tapping types.
As noted above, the side wall of each driven vane 6 in the Torx screw recess 2 is composed of the semicylindrical concave 3 and convex 4 continuing one to another. Therefore, it is difficult for conventional instruments to accurately measure the dimensional precision of such curved walls. Since only a minute clearance is allowed between the recess 2 of a smaller screw and a driver bit fitting therein, it is not easy to ensure a satisfactory preciseness to those recess 2 and bit.